Direct access storage devices (DASD) have become part of every day life, and as such, expectations and demands continually increase for greater speed for manipulating and holding larger amounts of data. To meet these demands for increased performance, the mechano-electrical assembly in a DASD device, specifically the Hard Disk Drive (HDD) has evolved to meet these demands.
In order for an HDD to hold more data, the magnetic recording heads as well as the disk media on which the data is written have undergone major advances in the past few years. A critical relationship between the head and disk is the spacing between their adjacent surfaces. This is typically known as the fly height.
The head flies above the disk by virtue of an air film created by the disk spinning next to a pattern on the surface of the slider (and magnetic recording transducer contained there within). This pattern on the slider is known as the Air Bearing Surface, or ABS. The ABS is fabricated on the surface of the slider that is closest to the disk. Typically the closest point on the ABS to the adjacent disk surface resides on the magnetic recording head. Typically the head resides at the end of the slider known as the trailing edge of the slider, so called the trailing edge because it is the last edge of the slider to fly over the disk. Once the slider is coupled to a suspension device the assembly is referred to as a Head Gimbal Assembly, or HGA. Other components such as a load beam, mount plate, and damper, which are well known to one skilled in the art, may also be coupled to the suspension and the assembly may still be referred to as an HGA.
Control of the fly height is critical to the density of data that can be written onto the disk surface. Fly height today is in the range of 5-15 nm. If heads fly too high, data might not be transferred to and from the disk with adequate amplitude, or signal strength. If heads fly too low, there exists the potential for catastrophic failure known as head crash. As the name implies, head crash is that situation which can occur when the head makes contact with the disk. This can result in either damage to the head, or to the disk, or to both. A head crash can result in loss of data and/or rendering the HDD inoperable.
The demand for more data storage is requiring the slider to operate with an ever-decreasing fly height and fly height tolerance. The margin has decreased for flying too high and having inadequate signal amplitude or flying too low and jeopardizing a head crash. This presents fly height control challenges.
As with any manufactured assembly, there are many tolerances and dimensions that affect the fly height of the head above the disk. One solution is to tightening the tolerances and distributions of those features of all components that effect fly height. However, many of the tolerances are associated with components that support the head or the disk and are very difficult and costly to control for the head manufacturer or disk manufacturer. Moreover, in many cases it is possible to have each distinct component that affects fly height in an HDD meet its individual dimension and tolerance, but when assembled with other components the resulting fly height of one or more sliders is unacceptable due to the interaction of tolerances. Conversely, many components scrapped for failing to meet the component specifications, may not fail the final HDD test if the components are properly matched during assembly. Therefore, screening components based on component specifications in most cases is very costly and may not be very effective.
There exists in the HDD industry a fly height control method whereby the magnetic recording transducer is urged closer to the disk recording media by means of a heating device imbedded in the slider. As the heating device is energized the material that surrounds the magnetic recording transducer expands and distends the magnetic recording device beyond the plane of the ABS. This fly height control method and apparatus is typically known as Thermal Fly-height Control, or TFC.
There are several disadvantages to TFC. The amount of displacement produced by TFC is small. TFC cannot absorb large deviations from the desired fly height. The reaction time for TFC is relatively slow. TFC relies upon the thermal expansion of the material that contains the magnetic recording transducer and therefore only distends the magnetic recording transducer once the surrounding material has become sufficiently warm. TFC only distends the magnetic recording transducer. It typically cannot retract the magnetic recording transducer beyond its initial relative position to the plane of the ABS. It would be advantageous to have a fly height control apparatus that addressed these disadvantages and limitations.